Electronic measurement and testing systems use relays to route analog signals. Switching devices used in these systems are required to have a very high off-resistance and a very low on-resistance. MOS analog switches have the disadvantage of non-zero leakage current and high on-resistance.
One example of a prior art microswitch is illustrated in FIG. 1. The basic structure is a micromechanical switch that includes a source contact 14, a drain contact 16, and a gate contact 12. A conductive bridge structure 18 is attached to the source contact 14. The bridge structure 18 overhangs the gate contact 12 and the drain contact 16 and is capable of coming into mechanical and electrical contact with the drain contact 16 when deflected downward. Once in contact with the drain contact 16, the bridge 18 permits current to flow from the source contact 14 to the drain contact 16 when an electric field is applied between the source and the drain.
Thus, as shown in FIG. 2, the voltage between the gate 12 and the source 14 controls the actuation of the device by generating an electric field in the space 20. With a sufficiently large voltage in the space 20, the switch closes and completes the circuit between the source and the drain by deflecting the bridge structure 18 downwardly to contact the drain contact 16.
Switches of this type are disclosed in U.S. Pat. No. 4,674,180 to Zavracky et al.; the entire contents of U.S. Pat. No. 4,674,180 are hereby incorporated by reference. In this device, a specific threshold voltage is required to deflect the bridge structure 18 so that it may contact the drain contact 16. Once the bridge 18 comes into contact with the drain contact 16, current flow is established between the source and the drain.
To obtain consistent performance the source must always be grounded, or the driving potential between the source and the gate must be floating relative to the source potential. However, this arrangement is not acceptable for many applications.
A preferred arrangement is a device with four external terminals instead of three: a source, a gate, and a pair of drain terminals, disposed such that a driving voltage between the gate and the source actuates the device, and establishes electrical contact between the drain electrodes, but keeps the drain electrodes electrically isolated from the source and gate electrodes. The advantage of this arrangement is that the current being switched does not alter the fields used to actuate the switch. Thus, the isolated contact completes a circuit independently from the circuitry used to actuate the switch. Several electrostatic microrelays of this type have been described in the prior art.
U.S. Pat. No. 5,278,368 to Kasano et al. discloses an electrostatic microrelay with a single-crystal silicon cantilever beam suspended above a gate electrode, and a contact bar attached to, but electrically isolated from, the underside of the beam. When the beam is actuated, the contact bar creates an electrical path between a pair of drain electrodes. Additional conductors distributed below and above the beam enable bistable operation. The manufacture of such a device requires the construction and alignment of several layers of conductors and insulators.
Yao and Chang (Transducers '95 Eurosensors IX, Stockholm, Sweden (1995)) have reported a similar device, with the difference that the cantilever beam is made of silicon oxide, and isolates the source from the beam contact without requiring an additional insulating layer.
Gretillat et al. (J. Micromech. Microeng. 5, 156–160 (1995)) have reported a microrelay with a polysilicon/silicon nitride/polysilicon bridge as the mechanical element.
U.S. Pat. No. 6,162,657 to Schiele, et al. disclosed a microrelay based on a gold cantilever sandwiched between silicon oxide layers to provide curvature to the beam by residual stress action, and hence improve isolation in the off-state.
A number of electromagnetically actuated microswitches and microrelays have been described in the prior art. The use of electromagnetic actuation limits the extent to which these devices can be miniaturized, and also results in higher power consumption than electrostatic actuation.
Another electrostatic microrelay is disclosed in U.S. Pat. No. 5,638,946 to Zavracky. As disclosed by Zavracky and illustrated in FIG. 3 of the present application, a micromechanical relay 28 includes a substrate 30 and a series of contacts (32, 34, 36) mounted on the substrate. The contacts include a source contact 32, a gate contact 34, and a drain contact 36. The drain contact 36 is made up of two separate contacts that are not shown in FIG. 3.
A beam 38 is attached at one end 40 to the source contact 32 and permits the beam to hang over the substrate 30. The entire beam structure 38, which comprises three separate components (a conductive body component 44 that includes the one end 40 attached to the source contact 32, an insulative element 42, and a conductive contact 46), is of sufficient length to overhang both the gate contact 34 and the drain contact 36.
As noted above, the beam structure 38 includes an insulative element 42 that joins and electrically insulates the conductive beam body 44 from the beam contact 46. The conductive beam body 44 overhangs only the gate contact 34. The insulative element 42 is of sufficient length to provide a mechanical bridge or extension between the conductive beam body 44 and the conductive contact 46 such that the conductive contact 46 overhangs the drain contact 36. In other words, the insulative element 42 provides additional lateral length to the beam structure 38.
In operation, actuation of the switch permits the beam contact 46 to connect the two separate contacts of the drain contact 36 and allow current to flow from one separate drain contact to the other.
The microrelay described above is based on a metallic cantilever beam. When a voltage is applied between the gate and the source electrodes, the electrostatic force between the beam and the gate electrode pulls the free end of the beam down. The free end or the beam contact is mechanically connected to, but electrically isolated from, the rest of the beam by a piece of insulating material, commonly a polyimide. When the beam is pulled down, a pair of contact bumps on the underside of the beam contact closes the path between a pair of thin film electrodes underneath the contact
The prior art device described above has some advantages relative to the other prior art devices referred previously. The device is fabricated from a single wafer and does not require wafer-bonding steps. It is fabricated using a surface micromachining process, which is generally simpler than a bulk micromachining process. The fabrication process is also a low temperature process relative to Si micromachining processes and traditional semiconductor fabrication processes. These advantages make it possible to build the device cheaply, and also make it feasible to integrate the device with semiconductor integrated circuits, with minimal interference with the semiconductor fabrication process.
However, a disadvantage of the device is that the material of the insulating segment 42 has to meet a number of requirements, some of which may be contradictory. It should electrically isolate the conductive beam contact 46 from the conductive beam body 44; it should have sufficient mechanical strength and rigidity to prevent excessive bending or breaking of the segment during actuation of the microrelay; it should have good adhesion to the beam body and the beam contact to ensure the mechanical integrity of the device when the microrelay opens and closes repeatedly; it should permit a method of deposition and patterning that is straight-forward and compatible with the rest of the fabrication process; and it should be chemically inert so that the microrelay can operate in a hermetic environment without being susceptible to contamination of the contacts by out-gassing from the insulating segment.
A practical embodiment of the device with the insulating segment 42 made out of a polyimide has been found to have poor mechanical integrity. More specifically, when the switch opens and closes repeatedly, the polyimide segment 42 loses adhesion with the conductive beam body 44 such that the insulative element 42 along with the conductive beam contact 46 fall off the end of the conductive beam body 44.
It is also possible that when the relay operates in a hermetic environment, the polyimide material will out-gas, particularly during high temperature cycles, and contaminate the microrelay context.
Therefore, it is desirable to design a microrelay wherein fewer requirements are imposed on the electrically insulating material, so that a microrelay with good electrical performance and mechanical integrity can be realized at low cost.